The mechanism of drug and hormone action on glycogenolysis and contractile function is being investigated in skeletal and cardiac muscle. The relative roles and interactions of cyclic AMP and Ca2 ion as second messengers in regulating phosphorylase a formation by isoproterenol is being studied in isolated hemidiaphragms incubated in an oxygenated bicarbonate-buffered physiological salt solution. At low Ca2 ion (less than 0.1mM) concentrations in the physiological salt solution, isoproterenol-stimulated phosphorylase a formation in muscles from C57BL mice was inhibited due to a reduced availability of Ca2 ion that is required for the activity of phosphorylase kinase. At high Ca2 ion (greater than 10 mM) concentrations, isoproterenol-stimulated phosphorylase a formation was inhibited due to a reduced adenylate cyclase activity. In studies on the binding of 125I-iodohydroxybenzylpindolol to the beta-adrenergic receptor and its displacement by nonradioactive ligands, high Ca2 ion concentrations affected agonists, but not antagonist, binding apparently by inhibiting conversion of the beta-adrenergic receptor from a state with a high affinity for agonists to one with a low affinity. Further studies will extend these investigations to pharmacogenetic variants carried in inbred strains of mice that differ in their EC50 (concentration giving one-half maximum response) or in their maximum response for isoproterenol-stimulated phosphorylase b to a conversion. These studies will, thus elucidate important information on the mechanisms of beta-adrenergic responses in intact mammalian skeletal muscles and the nature of genetic variation in these responses.